(1) Field of Technology
This technology relates to a bonding, sealing or joining technology, and more particularly to a method and apparatus suitable for brazing two materials together.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A hermetic seal, joint or bond to create a hermetic enclosure is a requirement of many microelectronic systems and high temperature electrochemical devices. For example, semiconductor lasers, oxygen generators, gas separation devices, fuel cells, chemical sensors and other microelectronic systems require metal and ceramic components to be hermetically bonded, joined or sealed to each other. Development of effective seals, joints and bonds has been one of the most critical areas of study for improving the performance of these devices.
However, creating the required air-tight or hermetic environment through a hermetic seal presents many challenges. The properties of the ceramics and metals themselves present the bulk of the problems. For example, hermetically sealing metal and ceramic components is a major issue in the design and manufacture of solid oxide fuel cell (SOFC) stacks. The strength of the seal may be tested in certain applications through exposure to an oxidizing and/or reducing atmosphere, operating temperatures of 450-850° C. and be subjected to thermal cycles. The device and specifically the seal must maintain its hermeticity along with its chemical and mechanical properties over typically several thousand hours or the lifetime of the device.
There are currently a number of accepted methods of creating a hermetic seal. However, all of them have disadvantages. For example in regards to SOFCs, particularly Planar Solid Oxide Fuel Cells (pSOFCs), Glass Joining (GJ), is commonly used when compared to other techniques. However, the seal is particularly susceptible to fracture when exposed to tensile stresses and is otherwise brittle, and nonyielding. Thermal expansion mismatch between glass and joining substrates is often encountered during nonequilibrium thermal events such as rapid stack heating or cooling.
Active Metal Brazing (AMB) is a popular method in joining ceramic-to-metal surfaces. AMB encourages wetting between the ceramic faying surface and the braze by utilizing a reactive element such as titanium. However, AMB presents at least two major problems for sealing solid-state electrochemical devices. (i) rapid deterioration of the joint at the ceramic-braze metal interface and an eventual loss of hermeticity due to the complete oxidization of the active species in the braze in high temperature operation, and (ii) the complex oxide materials in these devices are adversely affected by the typical processing conditions in AMB, namely exposure of the entire device to a reducing atmosphere greater than ˜800° C.
A relatively recent development, Reactive Air Brazing (RAB) is promising, although it has some disadvantages. RAB's distinct advantage is that the brazing may be directly conducted in an oxidizing atmosphere, typically in air at an elevated brazing temperature. The traditional braze material in RAB is a metal oxide-noble metal mixture. RAB produces a seal between a metal and ceramic part resistant to oxidization even at high temperatures. Although RAB has demonstrated some encouraging results, improvements addressing the design and preparation of the braze still need to be realized. For example, the presence of a continuous phase of CuO either within the interior of the braze, or along the braze/alumina interface is deleterious to the strength of the joint. Also, the wettability and bonding strength of the seal under traditional RAB is still relatively limited.
Thus, there exists a need for new methods of forming seals by advancing RAB technology through a novel material composition design and/or material processing to overcome these difficulties and produce metal to ceramic seals which function well in demanding conditions. The resulting development or invention of more advanced RAB is expected to provide a reliable high-temperature sealing, joining or bonding technology.
All referenced patents, applications and literatures are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. The invention may seek to satisfy one or more of the above-mentioned desire. Although the present invention may obviate one or more of the above-mentioned desires, it should be understood that some aspects of the invention might not necessarily obviate them.